JERUSALEM - An engineer in Gaza's power plant is being held in an Israeli prison after disappearing mysteriously off a train in the Ukraine last month, according to relatives, the U.N. refugee agency and an Israeli human rights group.

A spokesman for the Islamist group Hamas, which rules the Gaza Strip, called Friday on Ukraine to investigate the incident.

Dirar Abu Sisi, the operating manager of the only power plant in the Gaza Strip, went missing Feb. 19 after boarding a train in eastern Ukraine, where he had gone to apply for citizenship.

The Israeli rights group HaMoked, which assists Palestinian detainees, said it received confirmation this week from the Israeli Prison Service that Abu Sisi was being held in Shikma prison near the coastal city of Ashkelon, north of Gaza.

Abu Sisi's Ukrainian-born wife, Veronika, alleged in an interview with the Associated Press that he had been abducted by Israel's overseas intelligence service, Mossad, because of his key role in the power plant. A spokesman for the United Nations High Commissioner for Refugees in Ukraine said Abu Sisi was being held in Israel after what "looks like a violent abduction and not a legal extradition," AP reported.

Ghazi Abu Sisi, a brother of the engineer, said he had overseen a modification of the Gaza power plant to receive diesel fuel smuggled from Egypt, instead of from Israel, which had previously been the sole source of fuel for the power station.

Israeli officials have declined to comment on the case, citing a court-imposed gag order.